The use of a single resistance wire formed into a helical coil for use in electric resistance heating either for heating moving air, for radiant heating or for convection heating is well known in the prior art. In one type of heater, the resistance coils are energized to heat air passing over the coils, the heated air then being directed in a particular manner for heating purposes. One application using such a heater is an electric clothes dryer.
Examples of open coil heaters are found in U.S. Pat. Nos. 5,329,098, 5,895,597, and 5,925,273, all owned by Tutco, Inc. of Cookeville, Tenn. Each of these patents is incorporated by reference in its entirety herein. One type of an open coil electric resistance heater is a two stage heater described in U.S. Pat. No. 5,925,273. A side view of this type of heater is shown in FIG. 1 and designated by the reference numeral 10. The heater 10 has two heater elements 10a and 10b, optimally for use in a clothes dryer. The elements 10a and 10b are supplied with electricity via terminals 12 extending from the terminal block 28. The heater elements 10a, 10b are supported by a metal plate 14, which in turn supports a plurality of support insulators 16, which are well known in the art. The support insulators 16 support and isolate coiled portions of the elements, 10a and 10b, during operation of the heater.
The heater 10 includes opposing sidewalls (one shown as 6 in FIG. 1), wherein projections in the plate 14 extend through slots 20 in the sidewall 6 to allow the sidewalls to support the plate.
Each of the electric heater elements, 10a and 10b, is arranged in series of electrically continuous coils which are mounted on the plate 14 in a spaced-apart substantially parallel arrangement. Each heater assembly 10a and 10b is arranged substantially equally and oppositely on both sides of the plate. Crossover portions 22a and 22b of each heater element 10a and 10b are provided wherein each crossover links one coil of each of the elements mounted on one side of the plate 14 with another coil of the same element found on the other side of the plate. The plate 14 has several cutout portions (not shown) to provide adequate clearance for the crossover portions, 22a and 22b, and the anticipated drooping or sagging movement of such portions.
Electricity is supplied to the heater assembly through the terminal block 28. The heater elements, 10a and 10b, are arranged so that the terminal connector portions or wire leads 32 and 34 which extend from an end 38 of each of the mounted coil sections to the terminal block are as short as possible. This aids in eliminating or reducing the need for supporting the connector portions. For the longer runs, the wire leads, 32 and 34, are partially enclosed with an insulating member 36. The insulating member 36 may be formed from any type of insulating material suitable for this purpose, e.g., a ceramic type. The insulating member is generally tubular in shape and rigid.
One of the most significant aspects of the open coil heater design is that of routing the uncoiled connector portions extending from the ends of heater coils (resistance wire leads) so they are protected from contact with either adjacent live heater wire sections or from contact with bare metal, either of which creates a potential failure. The resistance wire leads for connecting to the termination point(s) for ultimate connection to a source of electrical power must therefore be routed and protected from dead metal and from electrically live parts of a different polarity than the resistance wire leads. When physical separation cannot be achieved using design techniques available to the technology, the prior art, as noted above, has used insulating tubes made of appropriate materials for the application, e.g., ceramic tubes are used for routing and isolating the resistance wires lead, see items 36 in FIG. 1.
However, the mere use of ceramic tubes alone to protect the resistance wire leads does not always solve all of the problems in these types of heaters. The manufacturing of open coil heaters that use insulating tubes to protect a length of bare resistance wire lead section often requires more than one tube section for each wire lead. That is, to span longer sections it is necessary to use multiple tube pieces. At the point where two tubes meet there is, of course, a narrow opening and when electrical spacing requirements per safety standards are considered, the opening can be or is significant. The same condition may exist at the free end of a ceramic tube, i.e. where there is no adjacent tube. Even under the best of conditions there are over-the-surface and through-the-air distance conditions occurring where an electric potential exists. This electrical potential could result in either a ground out or an electrical short should movement occur, both of which are dangerous conditions to be avoided.
There are practical reasons why multiple tubes may be required to span one section of bare resistance wire. The first is that longer sections of insulating tubes are more expensive to manufacture than shorter sections, so that using a number of shorter sections may be more economical.
Another reason that longer tubes are shunned is that shipping and handling relatively long tubes during heater assembly result in damage or breakage. Further it is often more economical to have available multiple short pieces that are of a lower cost per unit length in order to accommodate various length requirements than to have exact lengths made for every requirement. A given length requirement can be fulfilled by using two or more short pieces of insulating tubes to span the longer required distance.
Even when a single insulating tube is of sufficient length to span the distance from the end of the heating coil to the termination point, there may be lack of clearance at a tube end between the wire exiting the tube and adjacent dead metal. A further problem results should a tube break and the result is another potential electrical short or ground out condition being created.
It is also impractical to use an insulating tube with a wall thickness great enough to overcome the dangerous electrical conditions noted above. Thick walled tubes are costly to produce and handle during the manufacture of open coil heaters.
In the art of open coil heaters, separate tabs, clips, straps or stand-offs made of metal have been used to position and permanently restrain the insulating tubes containing resistance wires as described above. However, this method of restraint often creates mechanical stress resulting in tube breakage. Though a break in the tube in itself doesn't mean the wires will move from their intended routing, a possible electrical short or a ground condition may result as an effect of the wire contacting the above described metal restraining means.
In light of the shortcomings in protecting the bare wire leads or the problems when using ceramic leads of open coil electric resistance heaters, a need has developed for improved ways to minimize the possibility of shorting or grounding conditions. The present invention responds to this need by providing a standoff for use with the resistance wire leads or other uncoiled runs to minimize these grounding/shorting conditions.